Long-term exposure to elevated levels of air pollution has been linked to lung damage similar to that seen in smokers, according to a study published in the Journal of the American Medical Association. The study looked at the health effects of breathing in various pollutants, including the main component of smog, across six cities in the United States. “We found that an increase of about three parts per billion [of ground-level ozone] outside your home was equivalent to smoking a pack of cigarettes a day for 29 years,” Joel Kaufman, one author of the study, told NPR. The study involved nearly 7,000 adults that were exposed to annual average concentrations of between 10 and 25 parts per billion of ground-level ozone outside their homes. The researchers took up to five CT scans of each subject’s lungs over the past decade to measure changes, along with measuring the levels of pollution in the participants’ communities.